1. Field of the Invention
The present invention relates to an Asynchronous Transfer Mode (ATM) network system, and more particularly, to a method of converting one or more ATM cells in an ATM network system.
2. Discussion of the Related Art
In general, the ATM Adaptation Layer (AAL) provides support for higher-layer services such as signaling, circuit emulation, voice, and video. AALs also support packet-based services, such as IP, LANs, and frame relay.
There are four different types of the currently standardized AALs: AAL1, AAL2, AAL3/4, and AAL5. First of all, AAL2 supports ATM transport of connection-oriented Variable Bit Rate (VBR) packetized voice and video.
On the other hand, the Common Part (CP) AAL5 supports VBR traffic, both connection-oriented and connectionless. Support for connectionless or connection-oriented service is provided at the Service Specific Convergence Sublayer (SSCS) level. However, despite of the simplicity of AAL5, AAL5 is now coming under criticism for its relatively inefficient operation, especially, for the mix of packet sizes typically used on the Internet.
FIG. 1 illustrates an existing ATM network system for converting one or more AAL2 type ATM cells (hereinafter, “AAL2 cells”) to one or more AAL5 type ATM cells (hereinafter, AAL5 cell) or vice versa.
The system includes a first ATM cell converter 10 receiving one or more AAL2 cells and converting them into one or more AAL5 cells, and it further includes a second ATM cell converter 12 converting one or more AAL5 cells to one or more AAL2 cells. Lastly, the system includes an ATM switch 11 that switches each ATM cell (AAL5 cell) in an ATM level.
When the first ATM cell converter 10 receives one or more AAL2 cells, it extracts one or more Common Part Sublayer packets (CPS-Packets). Then it converts the extracted packets to one or more AAL5 cells and transmits the AAL5 cells to the ATM switch 11 in order switch each AAL5 cell in an ATM level. Next, the ATM switches the AAL5 cell and sends each AAL5 cell to the second ATM cell converter 12.
The ATM network system shown in FIG. 1 is used for transmitting a call related traffic message or signal control message in an IMT-2000 system including a Base Transceiver Station (BTS), a Radio Network Controller (RNC), and a Core Network (CN).
In the IMT-2000 system, BTS is connected to RNC through E1 (2.048 Mbps) line, and RNC is connected to CN through a Synchronous Transfer Module 1 (STM-1) line (155 Mbps). And a 53 bytes ATM cell transmits through those lines. In order to achieve higher efficiencies of the lines, the standards have decided to send a traffic message with AAL2 cells and a control message with AAL5 cells.
Namely, the standards have ruled that the traffic link interface and the signal link interface of the IMT-2000 should be AAL2 and AAL5, respectively. Even though a processor that handles both of the traffic and signal messages should be able to provide both of AAL2 and AAL5 protocols, such processor usually provides only a single AAL protocol. For this reason, some system developers often use a new type of the ATM cell having a system-preferred format.
The apparatus and method of converting one or more ATM cells in an ATM network system according to the prior art introduce several disadvantages. For converting N AAL5 cells including a user data set to AAL2 cells, the second ATM cell converter 12 must store all the AAL5 cells in a memory because the length-indicating field is included in the last AAL5 cell. Therefore, it requires a large amount of memory space and an unnecessary delay.
For converting AAL2 cells to AAL5 cells, similar problem will occur. When the first ATM cell converter 10 receives more than one AAL2 cells including a user data set, it initially generates more than one CPS packets. Thereafter, the first ATM cell converter 10 must store all the packets generated in order to convert them to AAL5 cells. This also requires a large memory space and an unnecessary delay for processing the cells in the system.